Lead II records the electrical potential from the right arm (negative) to the left leg (positive). It is the lead most commonly used for rhythm monitoring because the electrical axis closely parallels the normal direction of atrial and ventricular depolarization.

Why not the others?

B) Lead I = Right arm (−) → Left arm (+).
C) Reverse of Lead II (incorrect polarity).
D) The right leg is the ground (earth) electrode, not a recording electrode.
E) Lead III = Left arm (−) → Left leg (+).

High-yield summary:

Lead I: RA (−) → LA (+)
Lead II: RA (−) → LL (+)
Lead III: LA (−) → LL (+)

In lead II of the Einthoven triangle, which electrode configuration is used?